Prop Hunt is a game similar to that of Hide-and-Seek, only instead your character assumes the form of furnitures, objects and items, etc.
In Prop Hunt, there is a team of Props [RED] and a team of Hunters [BLUE]. In order for the props to win, all Hunters must die, or the game must run out of time. Glitches exist, such as a snow glitch in cs_office, and some people abuse them on servers to win rounds. In fact, some servers ban props that can get there, as well as rarely banning people who pick an extremely small prop, like a rock.
Some people go around a wall and entrance area to "juke" out Hunters, running between their legs and past them.
Prop Hunt has become a very popular gamemode even becoming so popular as to be found in other games like Minecraft, with gamemodes such as Block Hunt or Hide.
Garrys Mod Prop Hunt Part 1 - Super Filing Cabinet 3000